Ue capability dependent sync priority determination mechanism for v2x communication

ABSTRACT

A method for sidelink data retransmission can include receiving a first negative acknowledgement (NACK) at a base station (BS) from a receiver user equipment (Rx UE). The NACK corresponds to an original transmission of sidelink data over a sidelink from a transmitter user equipment (Tx UE). The method can further include selecting one or both of the BS and the Tx UE to perform a first retransmission of the sidelink data to the Rx UE. A method of sidelink synchronization can include selecting at a UE a timing reference with a highest priority among available sidelink synchronization timing references according to a rule indicating timing references listed from high priority to low priority: gNB or eNB; UE directly synchronized to gNB or eNB; UE in directly synchronized to gNB or eNB; GNSS; UE directly synchronized to GNSS; UE indirectly synchronized to GNSS; and remaining UEs.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of International ApplicationNo. PCT/CN2018/108378, “Advanced V2X Communication Mechanism” filed onSep. 28, 2018, and No. PCT/CN2018/108377, “UE Capability Dependent SyncPriority Determination Mechanism for V2X communication” filed on Sep.28, 2018, which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to wireless communications, andspecifically relates to sidelink communications for vehicularapplications and enhancements to cellular infrastructure.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Cellular based vehicle-to-everything (V2X) (e.g., LTE V2X or NR V2X) isa radio access technology developed by 3GPP to support advancedvehicular applications. In V2X, a direct radio link (referred to as asidelink) can be established between two vehicles. The sidelink canoperate under the control of a cellular system (e.g., radio resourceallocation) when the vehicles are within the coverage of the cellularsystem. Or, the sidelink can operate independently when no cellularsystem is present.

SUMMARY

Aspects of the disclosure provide a method for sidelink dataretransmission. The method can include receiving a first negativeacknowledgement (NACK) at a base station (BS) from a receiver userequipment (Rx UE). The NACK corresponds to an original transmission ofsidelink data over a sidelink from a transmitter user equipment (Tx UE).The method can further include selecting one or both of the BS and theTx UE to perform a first retransmission of the sidelink data to the RxUE.

In an embodiment, when the BS is selected to perform the firstretransmission of the sidelink data to the Rx UE, the sidelink data canbe transmitted to the Rx UE from the BS. When the Tx UE is selected toperform the first retransmission of the sidelink data to the Rx UE, afirst sidelink grant can be transmitted from the BS to the Tx UE for thefirst retransmission of the sidelink data to the Rx UE.

In an example, the selecting one or both of the BS and Tx UE can bebased on whether the BS has received the sidelink data of the originaltransmission. In an example, the method can further include executingpower control to the Rx UE such that the BS is able to receive the NACKfrom the Rx UE. In an example, the selecting one or both of the BS andTx UE is based on channel conditions of the sidelink between the Tx UEand the Rx UE, and a Uu link between the Rx UE and the BS, determined atthe Rx UE.

In an embodiment, the method can further include receiving a second NACKfrom the Rx UE corresponding to the first retransmission of the sidelinkdata to the Rx UE from the Tx UE, and selecting the BS to perform asecond retransmission of the sidelink data to the Rx UE. In an example,the method can further include receiving the sidelink data of theoriginal transmission from the Tx UE at the BS, receiving the sidelinkdata of the first retransmission from the Tx UE at the BS, andperforming soft combining of the sidelink data of the originaltransmission and the first retransmission at the BS.

In an embodiment, the method can further include transmitting from theBS a second sidelink grant included in a downlink control information(DCI) having a cyclic redundancy check (CRC) scrambled with a commonidentifier know to both the Tx UE and the Rx UE. The second sidelinkgrant indicates radio resources over the sidelink for the originaltransmission of the sidelink data. In an example, a number of time andfrequency resources for transmitting the second sidelink grant isdetermined based on a worse one of channel conditions of a first Uu linkbetween the Tx UE and the BS and a second Uu link between the Rx UE andthe BS.

Aspects of the disclosure provide a method for sidelink datatransmission. The method can include receiving a sidelink grant from aBS at a Rx UE indicating radio resources for transmission of sidelinkdata over a sidelink from a Tx UE to the Rx UE, and detecting at the RxUE the sidelink data transmitted over the sidelink at the radioresources indicated by the sidelink grant received from the BS.

Aspects of the disclosure provide a BS. The BS can include circuitryconfigured to receive a first NACK at the BS from a Rx UE, the NACKcorresponding to an original transmission of sidelink data over asidelink from a Tx UE, and select one or both of the BS and the Tx UE toperform a first retransmission of the sidelink data to the Rx UE. Whenthe BS is selected to perform the first retransmission of the sidelinkdata to the Rx UE, the sidelink data can be transmitted to the Rx UEfrom the BS. When the Tx UE is selected to perform the firstretransmission of the sidelink data to the Rx UE, a first sidelink grantcan be transmitted from the BS to the Tx UE for the first retransmissionof the sidelink data to the Rx UE.

Aspects of the disclosure provide a method of sidelink synchronization.The method can include selecting at a UE a timing reference with ahighest priority among available sidelink synchronization timingreferences according to a sidelink synchronization source priority rulethat includes different types of sidelink synchronization timingreferences each having a priority, and determining at the UE atransmission timing according to the selected timing reference. When thedifferent types of sidelink synchronization timing references are gNB oreNB based timing references, the different types of sidelinksynchronization timing references include the following types ofsidelink synchronization timing references listed from high priority tolow priority:

-   -   P0′: gNB or eNB,    -   P1′: UE directly synchronized to gNB or eNB,    -   P2′: UE in directly synchronized to gNB or eNB,    -   P3′: global navigation satellite system (GNSS),    -   P4′: UE directly synchronized to GNSS,    -   P5′: UE indirectly synchronized to GNSS,    -   P6′: remaining UEs.

In an example, when the different types of sidelink synchronizationtiming references are GNSS-based references, the different types ofsidelink synchronization timing references include the following typesof sidelink synchronization timing references listed from high priorityto low priority:

-   -   P0: GNSS,    -   P1: UE directly synchronized to GNSS,    -   P2: UE indirectly synchronized to GNSS,    -   P3: remaining UEs.

In an embodiment, the selecting includes receiving a sidelinksynchronization signal (SLSS) carrying information of a slot index. Inan example, a number of bits representing the slot index depends on anumerology of the SLSS. In an example, the SLSS includes a sidelinksecondary synchronization signal (S-SSS) and a demodulation referencesignal (DMRS), and a sequence of the S-SSS, a sequence of the DMRS, orthe sequences of the S-SSS and the DMRS in combination, represent theslot index. In an example, information of the slot index and a subframeindex is combined as one field and carried in the SLSS.

In an embodiment, the selecting includes receiving a SLSS carryingsidelink synchronization source priority information, wherein thesidelink synchronization source priority information is carried insignals included in the SLSS other than a sidelink physical broadcastchannel (PBCH).

In an embodiment, the selecting includes receiving a SLSS carryingsidelink synchronization source priority information in a sidelink PBCH.Bits of the sidelink synchronization source priority information arearranged at input bit positions of a polar encoder when information bitsof the sidelink PBCH are encoded with the polar encoder, such that thebits of the sidelink synchronization source priority information can beobtained without fully decoding the sidelink PBCH.

In an embodiment, the method for sidelink synchronization furtherincludes receiving a sidelink identifier (ID) for identifying sidelinkunicast, groupcast, or broadcast, and performing a sidelink transmissionusing the sidelink ID scrambled with a cell ID of a serving cell whenthe UE is within or out of coverage of the serving cell.

Aspects of the disclosure provide a UE for sidelink synchronization. TheUE can include circuitry configure to select a timing reference with ahighest priority among available sidelink synchronization timingreferences according to a sidelink synchronization source priority rulethat includes different types of sidelink synchronization timingreferences each having a priority, and determine a transmission timingaccording to the selected timing reference. When the different types ofsidelink synchronization timing references are gNB or eNB based timingreferences, the different types of sidelink synchronization timingreferences include the following types of sidelink synchronizationtiming references listed from high priority to low priority:

-   -   P0′: gNB or eNB,    -   P1′: UE directly synchronized to gNB or eNB,    -   P2′: UE in directly synchronized to gNB or eNB,    -   P3′: GNSS,    -   P4′: UE directly synchronized to GNSS,    -   P5′: UE indirectly synchronized to GNSS,    -   P6′: remaining UEs.

Aspects of the disclosure provide a non-transitory computer-readablemedium storing a program. The program is executable by a processor toperform the method of sidelink synchronization.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1A and FIG. 1B show two data retransmission processes forretransmitting data previously transmitted in a sidelink communication;

FIG. 2 shows a cluster of UEs with a base station present according toan embodiment of the disclosure;

FIG. 3 shows another cluster of UEs without presence of a base stationaccording to an embodiment of the disclosure;

FIG. 4 shows an example of a gNB/eNB-based synchronization ruleaccording to an embodiment of the disclosure;

FIG. 5 shows a GNSS-based synchronization rule according to anembodiment of the disclosure;

FIG. 6-FIG. 13 show tables of sidelink synchronization source priorityrules according to some embodiments of the disclosure;

FIG. 14 shows a sidelink data transmission process according to anembodiment of the disclosure;

FIG. 15 shows another sidelink data transmission process according to anembodiment of the disclosure;

FIG. 16 shows a sidelink synchronization process according to anembodiment of the disclosure; and

FIG. 17 shows an exemplary apparatus according to embodiments of thedisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1A and FIG. 1B show two data retransmission processes 130 and 140for retransmitting data previously transmitted in a sidelinkcommunication. One of those two data retransmission processes 130 and140 can be adaptively selected for data retransmission, for example,depending on channel conditions.

FIG. 1A shows a wireless communication system 100 according to anembodiment of the disclosure. The system 100 can include a base station(BS) 101, a first user equipment (UE) 102, and a second UE 103. The BS101 can be an implementation of a gNB specified in the 3rd GenerationPartnership Project (3GPP) New Radio (NR) standards, or can be animplementation of an eNB specified in 3GPP Long Term Evolution (LTE)standards. Accordingly, the BS 101 can communicate with the UE 102 or103 via a radio air interface 110 (referred to as a Uu interface 110)according to respective wireless communication protocols. Alternatively,the BS 101 may implement other types of standardized or non-standardizedradio access technologies, and communicate with the UE 102 or 103according to the respective radio access technologies. The UE 102 or 103can be a vehicle, a computer, a mobile phone, and the like.

The UE 102 and UE 103 can communicate with each other based onvehicle-to-everything (V2X) technologies specified in 3GPP standards. Adirect radio link 120, referred to as a sidelink (SL), can beestablished between the UEs 102 and 103. The UE 102 can use a samefrequency for uplink transmissions over a Uu link 111 and SLtransmissions over the SL 120. Similarly, the UE 103 can use a samefrequency for uplink transmissions over a Uu link 112 and SLtransmissions over the SL 120. In addition, allocation of radioresources over the SL120 can be controlled by the BS 101.

In an embodiment, the system 100 implements a hybrid automatic repeatrequest (HARQ) scheme for data retransmission. A first HARQ dataretransmission process 130 of this HARQ scheme is illustrated in FIG.1A. The process 130 can include steps from S131 to S136. During theprocess 130, the UE 102 transmits SL data, while the UE 103 receives theSL data. Accordingly, the UE 102 is referred to as a transmitter UE (TxUE) while the UE 103 is referred to as a receiver UE (Rx UE).

At S131, a first SL grant is transmitted from the BS 101 to the Tx UE102 over the Uu link 111. The first SL grant may indicate radioresources for the data transmission from the Tx UE 102 to the Rx UE 103.The first SL grant may further indicate radio resources for positive ornegative acknowledgement (ACK/NACK) feedback from the Rx UE 103 to theTx UE 102. Alternatively, the radio resources for ACK/NACK can bedetermined based on the radio resources allocated for the datatransmission. For example, by configuration, the frequency and timedomain location of the radio resources for ACK/NACK feedback can bedetermined based on that for the data transmission. In an example, thetransmission of the first SL grant is performed as a response to receivea scheduling request from the Tx UE 102.

At S132, the SLdata is transmitted from the Tx UE 102 to the Rx UE 103over the SL 120 based on the first SL grant received at S131. Forexample, using the radio resource indicated by the first SL grant, theTx UE 102 may transmit a physical SL control channel (PSCCH) followed bya physical SL share channel (PSSCH) over the SL 120. The PSCCH carriesinformation scheduling the PSSCH. The Rx UE 103 can detect the PSSCHbased on the scheduling information carried in the PSCCH.

The signal for transmitting the SL data can be a broadcast signal, or abeamformed signal, towards both the Rx UE 103 and the BS 101 over thechannel shared between the uplink of the Uu link 111 and the SL 120. Inaddition, the allocation of the radio resource for the SL datatransmission is provided by the BS 101, and thus is known to the BS 101.If the BS 101 executes proper power control over the Tx UE 102, and achannel condition over the shared wireless channel is good enough, theBS 101 can detect and decode the SL data transmitted from the Tx UE 102.

At S133, the reception of the transmitted SL data is failed at the Rx UE103, and a NACK is transmitted from the Rx UE 103 over the SL 120.Similarly, the signal for the transmission of the NACK can be detectedby both the Tx UE 102 and the BS 101. Alternatively, the signal for thetransmission of the NACK is detected by at least the BS 101 in otherexamples. For example, due to poor channel condition of the SL 120, theRx UE 102 may not detect the NACK feedback.

At S134, a second SL grant for retransmission of the SL data istransmitted from the BS 101 to the Rx UE 102 over the Uu link 111. Forexample, in response to receiving the NACK for the SL data transmissionfrom the Rx UE 103, the BS 101 can determine to transmit the second SLgrant. Similarly, radio resources for ACK/NACK feedback can be indicatedby or derived from the second SL grand.

At S135, the SL data is retransmitted from the Tx UE 102 to the Rx UE103 based on the second SL grant over the SL 120. In alternativeexamples, the first SL grant may specify radio resources for theretransmission of the SL data. Under such a configuration, the Tx UE 102may retransmit the SL data if the Tx UE 102 receives the NACK feedbackover the SL 120. Accordingly, S134 may be skipped in those examples.

At S136, when the detection of the SL data is successful at the Rx UE103, an ACK for the data retransmission can be fed back to the Tx UE 102over the SL 120. Similarly, the signal for the ACK transmission may alsoreach the BS 101. The first HARQ data retransmission process 130 can becompleted after S36.

FIG. 1B shows the same wireless communication system 100 as in FIG. 1A.The system 100 still implements the HARQ data retransmission scheme.However, a second HARQ data retransmission process 140 of the HARQ dataretransmission scheme takes place in the system 100. Different from thefirst process 130 in FIG. 1A where the Tx UE 102 performs the SL dataretransmission, in the second process 140, the SL data retransmission isperformed by the BS 101. Specifically, the second process 140 caninclude steps from S141 to S145.

The steps of S141 to S143 can be similarly performed as in the steps ofS131 to S133. However, different from the first process 130, at S144,the BS 101 may determine to use the BS 101 to perform the SL dataretransmission instead of the Tx UE 102. For example, instead of sendingthe second SL grant to trigger the Tx UE 102 to perform the SL dataretransmission over the SL 120, the BS 101 may transmit the SL data(that is previously received at S142) via the Uu link 112. For example,the BS 101 may transmit downlink control information (DCI) on a physicaldownlink control channel (PDCCH) that schedules a physical downlinkshared channel (PDSCH) carrying the SL data. The Rx UE 103 may detectthe DCI and subsequently detect and decode the SL data.

At S145, when the detection is successful, the Rx UE 103 may transmit anACK for the SL data retransmission over the Uu link 112 as a response tothe SL data retransmission over the Uu interface 110 at S144. Forexample, the ACK information may be carried on a physical HARQ indicatorchannel (PHICH) or a physical uplink shared channel (PUSCH). Those radioresources of the PHICH or the PUSCH can be indicated by or derivedaccording to the DCI received at S144.

Under such a configuration, the Tx UE 102 may not detect the ACKtransmitted over the Uu interface 110. In an example, the Tx UE 102 mayreceive anew SL grant from the BS 101 carrying a new data indicator. Inresponse to receiving the latest SL grant carrying the new dataindicator, the Tx UE 102 may understand the SL data has been transmittedsuccessfully, and accordingly release the SL data from a buffer.

The second HARQ data retransmission process 140 can be completed afterS145.

According to an aspect of the disclosure, by implementing the HARQ dataretransmission scheme, the BS 101 can adaptively select the BS 101 orthe Tx UE 102, or both, to perform HARQ retransmission for unicast orgroupcast communications over the SL 120. For example, depending on adecision of the BS 101, one of the processes 130 or 140 can beperformed. The decision of selecting one or both of the BS 101 or the TxUE 102 to perform the HARQ retransmission can depend on the flowingconsiderations.

In an embodiment, the selection of one or both of the BS 101 and the TxUE 102 for the SL data retransmission can first be based on whether theSL data is received (e.g., successfully decoded) at S132 or S142, andwhether the NACK is received (e.g., successfully decoded) at S133 orS143. For example, due to poor channel conditions, the BS 101 may notreceive the SL data or the NACK. When the SL data is not received, theBS 101 can select the Tx UE 102 to perform the SL data retransmission.When the SL data is received but the NACK is not received, the BS 101may not perform retransmission operations either from the BS 101 or theTx UE 102. Alternatively, the BS 101 may be configured to blindlyperform a retransmission via either the BS 101 or the Tx UE 102, orboth, when the ACK or NACK is not received at a scheduled time.

In an embodiment, in order to receive the SL data and the ACK/NACK atthe BS 101, the BS 101 can be configured to execute power control fortransmissions by the Tx UE 102 and the Rx UE 103. For example, the powercontrol to the Tx UE 102 can be based on a pathloss of the worse onebetween the Uu link 111 and the SL 120, and capped by a maximum allowedpower level applied to the Uu link 111. For example, the Rx UE 103 canperform measurement periodically for signals received from the Tx UE 102over the SL 120, and report measurement results to the BS 101. The BS101 may obtain measurement results of signals received from the Tx UE102. Based on those measurement results, the BS 101 can have knowledgeof the path loss over the Uu link 111 (uplink direction) and the SL 120(for transmissions from the Tx UE 102). The BS 101 can accordinglytransmit a DCI indicating a power adjustment to the Tx UE 102 to controltransmission power of the Tx UE 102.

To avoid interference over the Uu interface 110, the Tx UE 102 mayincrease the transmission power with a restriction of the maximumallowed power level for the Uu link 111. In an embodiment, the maximumallowed power level configured for the Uu link 111 is signaled from theBS 101 to the Tx UE 102.

In an embodiment, the BS 101 can be configured to separately executepower control to the SL 120 and the Uu link 111. For example, differentDCIs with different radio network temporary identifiers (RNTIs) can betransmitted from the BS 101 to the Tx UE 102. The different RNTIs can beused to differentiate the different DCIs that carry different poweradjustments applicable to transmissions over the SD link 120 or the Uulink 111. Similarly, the transmission power over the SL 120 can becapped by the maximum allowed power level configured for the Uu link111.

In an embodiment, the selection of one or both of the BS 101 and the TxUE 102 for the SL data retransmission can further be based on channelconditions in the SL 120 and the Uu links 111 or 112. For example, thechannel conditions can include channel state information (CSI) and pathloss in the SL 120 and the Uu links 111 or 112. For example, the BS 101can obtain the channel conditions of the SL 120 and the Uu link 112 byreceiving a report from the Rx UE 103 that performs a relatedmeasurement process. In an example, when a channel quality of the SL 120(from the Tx UE 102 to the Rx UE 102) indicated by the channelconditions is better than a channel quality of the Uu link 112 (downlinkdirection), the BS 101 may select the Tx UE 102 for the SL dataretransmission. Otherwise, the BS 101 may select the BS 101 itself forthe SL data retransmission.

In an example, the BS 101 can compare the channel conditions of the SL120 and the Uu link 112 with a threshold, or separate thresholds. Whenthe channel quality of the SL 120 or the Uu link 112 is above therespective threshold, one or both of the SL 120 and the Uu link 112 canbe selected for the SL data retransmission.

In an embodiment, the selection of the BS 101 or the Tx UE 102 for theSL data retransmission can be based on a reliability or QoS requirementassociated with each of different types of SL data. For example, givencertain channel conditions of the SD link 120 and the Uu links 111 or112, the BS 101 may make different selections depending on the types ofthe SL data, for example, to satisfy the reliability or QoS requirementsof certain types of DL data.

In an embodiment, the selection of the BS 101 or the Tx UE 102 for theSL data retransmission can be performed in the following way. The BS 101may first select the Tx UE 102 to perform the SL data retransmission.When reception of the retransmitted SL data is failed for a second time(e.g., the BS 101 receives a NACK from the Rx UE 103 for a second time),the BS 101 may determine to perform a next SL data retransmission usingthe BS 101.

Under such a configuration, the BS 101 may be configured with a softcombining buffer for storage of SL data received from the Tx UE 102 inthe successive occasions (the original transmission and theretransmission of the Tx UE 102). Those two pieces of SL data can besoft combined (e.g., with a chase combining or incremental redundancyscheme) and decoded, and subsequently used for the next SL dataretransmission from the BS 101.

In an embodiment, the SL grant (e.g., transmitted at S132, S134, or S142in the process 130 or 140) can be received by both the Tx UE 102 and theRx UE 103. For example, the Uu links 111 and 112 operate on a samefrequency layer (e.g., a same cell). The SL grant can be carried in aDCI having a cyclic redundancy check (CRC) scrambled with a commonidentifier (ID) assigned to both the Tx UE 102 and the Rx UE 103. Forexample, the common ID can be an RNTI or a physical layer ID known toboth the Tx UE 102 and the Rx UE 103. Thus, when the DCI is transmittedin the downlink direction over the Uu interface 110, both the Tx UE 102and the Rx UE 103 can detect the DCI to obtain the DL grant. In thisway, both the Tx UE 102 and the Rx UE 103 can know the radio resourcesgranted for transmission of the SL data over the SL 120. Transmission ofthe PSCCH (that schedules the PSSCH) over the SD link 120 as performedin S132, S135, or S142 can be omitted to save radio resources and reduceSL data transmission complexity.

In one example, the BS 101 determines a number of time and frequencyresources for the SL grant based on the worse one between the Uu link111 and the Uu link 112, such that both the Tx UE 102 and Rx UE 103 canreceive the SL grant. For example, the one of the Uu links 111 and 112having a worse channel condition (e.g., CSI, pathloss) is used todetermine a the number of time and frequency resources for transmittingthe SL grant provided by the BS 101 to both Tx UE 102 and the Rx UE 103.

In an embodiment, two different methods can be used for the Rx UE 103transmitting the ACK/NACK feedback over the SL 120 for reception the DLdata from the Tx UE 102. The first method is to transmit the ACK/NACKfeedback using the radio resources configured by or derived from the SLgrant, for example, in a form of a physical SL HARQ indicator channel.The second method is to attach the ACK/NACK feedback to a PSSCHtransmitted from the Rx UE 103 to the Tx UE 102. For example, when theRx UE 103 has SL data to be transmitted to the Tx UE 102, a PSSCH can betransmitted over the SL 120 from the Rx UE 103 to the Tx UE 103. TheHARQ feedback information can be included in the PSSCH.

In an embodiment, when there is SL data stored in a buffer waiting to betransmitted to the Tx UE 102, the Rx UE 103 transmits a PSSCH thatcarries SL scheduling request related information to the Tx UE 102. TheSL scheduling request related information can include a SL schedulingrequest, buffer status information (indicating SL data in a buffer atthe Rx UE 103), and/or traffic type (e.g., unicast, groupcast, orbroadcast). In an embodiment, instead of carrying the SL schedulingrequest related information in a PSSCH, the Rx UE 103 may include the SLscheduling request related information in a media access control (MAC)control element (CE) of a transport block carried in a PSSCH.

In an embodiment, at the Tx UE 102, the Tx UE 102 can transmit a PSSCHto the Rx UE 103 that includes radio resource allocation information forSL date transmission and/or reception between the Tx UE 102 and the RxUE 103. In an example, the transmission of the radio resource allocationinformation can be a response to and based on SL scheduling relatedinformation (e.g., a scheduling request, buffer status, and/or traffictype) received from the Rx UE 103.

For example, the radio resource allocation information may include aradio resource configuration for the SL data transmission and/orreception between the Tx UE 102 and Rx UE 103 during a period. Forexample, the configuration can specify a sequence of periodicaltransmission occasions over the period. Alternatively, the radioresource allocation information may indicate radio resources for thetransmission and/or reception between the Tx UE 102 and Rx UE 103 in anext time unit (e.g., a slot). Based on the radio resource allocationinformation carried in the PSSCH, the Tx UE 102 and Rx UE 103 can have amutual understanding of when the respective transmissions and receptionswill take place between the Tx UE 102 and Rx UE 103. Collisions of SLdata transmission of two opposite directions over the SL 120 can thus beavoided.

In an example, the above scheme of carrying radio resource allocationinformation over a PSSCH can be applied to UEs forming a cluster. Thecluster may include a master UE that transmits radio resource allocationinformation over a PSSCH to other member UEs of the cluster. In anexample, the master UE is selected by a BS. In other examples, themaster UE can be determined without a BS.

In various embodiments, the above described examples of carrying radioresource allocation information over a PSSCH, carrying a SL schedulingrequest over a PSSCH or a MAC CE, or carrying ACK/NACK information overa PSSCH, can be performed by UEs within a coverage of a BS or out of acoverage of a BS. For example, when a UE is out of coverage of a BS, theUE can determine SL radio resource allocation without control of the BS.When a UE is within the coverage of a BS, SL radio resource allocationmay or may not be controlled by the BS.

FIG. 2 shows a cluster 200 of UEs 201-205 according to an embodiment ofthe disclosure. Each UE 201-205 synchronizes to a nearby synchronizationsource and accordingly determines a transmission timing or a receptiontiming for sidelink communications (e.g., unicast, groupcast, orbroadcast) with nearby UEs within the cluster 200. A synchronizationsource 210 (e.g., a gNB, an eNB, or a global navigation satellite system(GNSS)) (or a synchronization signal (SS) from the synchronizationsource 210) is used as a top priority timing reference which is extendedto the UEs 201-205 within the cluster 200.

For example, the UEs 201-202 are within the coverage of thesynchronization source 210, and accordingly can directly synchronize tothe synchronization source 210. For example, a gNB or eNB mayperiodically transmit LTE or NR synchronization signals (SSs) such asprimary synchronization signal (PSS), secondary synchronization signal(SSS), and physical broadcast channel (PBCH) signal. GNSS satellites maycontinuously transmit navigation signals. Using those signals as timingreferences, the UEs 201-202 can obtain the reference timing, andaccordingly determine the transmission or reception timing of itself.

After being synchronized to the synchronization source 210, the UE 202can transmit a sidelink synchronization signal (SLSS) that issynchronized to the synchronization source 210. The SLSS can include asidelink primary synchronization signal (S-PSS), a sidelink secondarysynchronization signal (S-SSS), and a sidelink physical broadcastchannel (S-PBCH, or PSBCH) signal, and can be transmitted periodically.In an example, when the UE 202 starts to transmit the SLSS can becontrolled by a gNB or an eNB which the UE 202 is connected to or campedon. In an example, the UE 202 itself can make a decision when totransmit the SLSS. For example, the UE 202 can determine to transmit theSLSS when a quality (e.g., indicated by reference signal received power(RSRP)) of a signal from the gNB or the eNB is below a threshold.

By receiving the SLSS from the UE 202 as a timing reference, the UEs 203and 205, which are out of the coverage of the top priority timingreference 210, can synchronize to the UE 202, and becomes indirectlysynchronized to the top priority synchronization source 210.

Similarly, the UE 203 can transmit a SLSS that is synchronized to thetiming reference of the UE 202. By using the SLSS of the UE 203 as atiming reference, the UE 204 can by synchronized to the UE 203.

FIG. 3 shows another cluster 300 of UEs 301-304 according to anembodiment of the disclosure. Each UE 301-304 synchronizes to a nearbysynchronization source in order to determine a transmission timing or areception timing for sidelink communications (e.g., unicast, groupcast,or broadcast) with nearby UEs within the cluster 300. In the cluster300, none of the UEs 301-304 is within the coverage of a synchronizationsource (e.g., the synchronization source 210 in the FIG. 2 example). Forexample, when the UE 301 is powered on or has lost synchronization toother synchronization sources (e.g., a gNB, an eNB, a GNSS, or a UE),the UE 301 tries to search for a synchronization source (e.g., a gNB, aneNB, a GNSS, or a UE) and is not successful. Accordingly, the UE 301 mayautonomously determine a transmission timing, and transmit a SLSS basedon this transmission timing. The UE 302 can use the SLSS from the UE 301as a timing reference and determine a transmission timing of the UE 302.In a similar way, the UEs 303-304 can perform synchronization based on aSLSS transmitted from the UE 302.

According to an aspect of the disclosure, in order to facilitatesynchronization operations in sidelink communications, a synchronizationrule can be specified. According to the synchronization rule, differenttiming references (or synchronization sources) can be categorized intodifferent groups or types. Each type of the timing references is given apriority. A synchronization configuration indicating the synchronizationrule can be configured to a UE. Based on this synchronizationconfiguration, the UE can select a timing reference with the highestpriority among available timing references to synchronize with.

In an embodiment, the synchronization configuration can be configured tothe UE through radio resource control (RRC) signaling or broadcastedsystem information when the UE is in coverage of a BS. The UE can storethe synchronization configuration locally, and use the synchronizationconfiguration when being in an in-coverage status (connected to a BS inRRC connected mode or camping on a BS in RRC idle mode), or in anout-of-coverage status. Alternatively, the synchronization configurationcan be configured to the UE, for example, by storage in a subscriberidentity module (SIM) or in a non-volatile memory of the UE. Forexample, when the UE is out of coverage, and has not received asynchronization configuration from a BS before, the UE can use thelocally stored synchronization configurations to select a timingreference. After the UE receives the synchronization configuration fromthe BS, the UE can use the received synchronization configuration. In anexample, the UE can use the locally stored synchronization configurationwhen the UE is in coverage or out of coverage of a BS.

FIG. 4 shows an example of a gNB/eNB-based synchronization rule 400according to an embodiment of the disclosure. The gNB/eNB-basedsynchronization rule 400 includes 7 types of timing references withpriorities from P0′ to P6′. The 7 types of timing references are listedin priority decreasing order.

In an example, a current UE configured (e.g., by signaling or by storageat the UE) with the rule 400 can perform a synchronization procedure inthe following way. As the gNB or eNB is configured to be the toppriority timing reference, the current UE can first search for a SS froma gNB or eNB, for example, after powered on or losing synchronization.If a SS from a gNB or eNB with a signal quality above a threshold isfound, the current UE can synchronize to the SS. In an example, whenboth a gNB and an eNB are found, the one of the respective SSs with ahigher signal quality (e.g., RSRP) is used as the timing reference.

Using a gNB or an eNB as a top priority timing reference, UEs within acoverage of a gNB or eNB can be synchronized to a same BS (or a samecell). As a result, sidelink transmissions among the UEs will take placewithin intended time-frequency resources, thereby reducing uncontrolledinterference to other sidelink and non-sidelink (cellular uplink)transmissions in a same band.

When no suitable SS from a gNB or eNB is found, the current UE cancontinue to search for a SLSS from a UE directly synchronized to a gNBor an eNB. For example, in the FIG. 2 example, assuming thesynchronization source 210 is a gNB, the UE 205 at the current locationcan receive SLSSs from both the UE 202 and the UE 203. Based on thereceived SLSSs, the UE 205 can know the UE 202 is a UE directlysynchronized to the gNB (the synchronization source 20), while the UE203 is a UE indirectly synchronized to the gNB. According to thesynchronization rule 400, the UE 205 can select the UE 202 as a timingreference, as the UE 202 has a priority of P1′ while the UE 203 has apriority of P2′ as specified in the rule 400.

When a timing reference is extended through a chain of UEs, timingerrors can accumulate with respect to the original synchronizationsource. For example, the UE 202 (that is a hop 0 timing reference (e.g.,direct synchronization)) has a higher timing accuracy than the UE 203(that is a hop 1 timing reference (e.g., indirect synchronization)).Accordingly, the UE 205 can obtain a transmission or reception timingwith a higher accuracy from the UE 202 than from the UE 203.

In an embodiment, information of a number of hops of a UE with respectto a top priority synchronization source can be carried in a SLSS of therespective UE. The current UE can accordingly select the one with thefewest hops among available UEs (e.g., UE indirectly synchronized to agNB or an eNB) as a sidelink synchronization source.

When no UE indirectly synchronized to a gNB or an eNB is found,according to the synchronization rule 400, the current UE can search fornavigation signals of a GNSS (with a priority of P3′). If no GNSS isavailable, the current UE can search for a UE directly synchronized to aGNSS (a UE with a priority of P4′) to synchronize with. If no UEdirectly synchronized to a GNSS is available, the current UE may searchfor a UE indirectly synchronized to a GNSS (a UE with a priority ofP5′). If no UE indirectly synchronized to a GNSS is found, the currentUE may determine to use one of other available UEs as a timing reference(a UE with a priority of P6′). For example, those other available UEscan belong to a cluster of UEs where no gNB, eNB, or GNSS is present,such as the UEs 301-304 in the FIG. 3 example.

When no UEs are available around the current UE, the current UE canautonomously determine a transmission timing and accordingly transmit aSLSS. Please note that according to different designs, the orders can bedifferent from the embodiment shown in FIG. 4.

In some embodiments, when the synchronization source priority rule 400is configured to a UE (e.g., by signaling or by storage in a SIM), theUE may perform a sidelink synchronization according to the rule 400,however, with consideration of UE capability restriction.

For example, a NR UE is able to receive a SS from a NR BS (e.g., gNB),but may not be able to receive a SS from an LTE BS (e.g., eNB). Incontrast, an LTE UE is able to receive SS from an LTE BS but may not beable to receive SS from a NR BS.

For example, a NR V2X UE has the ability to perform sidelinkcommunications according to protocols specified by a NR V2X standard,and accordingly can perform synchronization using a SLSS compliant withthe NR V2X standard. Accordingly, the NR V2X UE can receive a SLSS froman LTE UE or NR UE that operates according to the NR V2X standard.However, the NR V2X UE may not use a SLSS from a UE operating accordingto the LTE V2X standard.

Similarly, an LTE V2X has the ability to perform sidelink communicationsaccording to protocols specified by the LTE V2X standard, andaccordingly can perform synchronization using a SLSS compliant with anLTE V2X standard, but may not perform synchronization suing a SLSS ofthe NR V2X standard.

Because of different UE capabilities, available synchronization sourcesfor different UEs can be different when a same set of synchronizationsources is present. Accordingly, in an embodiment, a UE configured withthe synchronization source priority rule 400 can derive asynchronization source priority configuration or rule according to therule 400 based on a capability of the UE. For example, some kinds ofsynchronization sources unusable for the UE may not be included in thederived rule or configuration.

In an embodiment, instead configure the rule 400 to a UE, asynchronization source priority configuration or rule can first bederived based on the rule 400 and a capability of the UE, andsubsequently configured to the UE (e.g., by signaling or by storage atthe UE).

FIG. 5 shows a GNSS-based synchronization rule 500 according to anembodiment of the disclosure. In the GNSS-based synchronization rule500, timing references are categorized into 4 groups or types each witha priority. The 4 types of timing references are listed in prioritydecreasing order from P0 to P3. Different from the rule 400, the rule500 includes the GNSS as a top priority synchronization source.Different network operators may prefer different network deploymentstrategies and thus may choose the GNSS or the gNB and/or eNB as the toppriority sidelink synchronization source. When the GNSS is preferred,the rule 500 can be used.

FIG. 6-FIG. 13 show tables 600-1300 of sidelink synchronization sourcepriority rules according to some embodiments of the disclosure. Thosepriority rules can be configured to a UE (e.g., by RRC or systeminformation signaling, or storage in a SIM module (e.g., a universalintegrated circuit card (UICC) module or a memory), and used as a basisfor selecting a timing reference.

Each table can include 2 or 3 gNB-based, eNB-based, or GNSS-basedpriority rules. For example, depending on network operator's deploymentpreference, the gNB-based, eNB-based, or GNSS-based priority rules canbe configured to a UE and used as a basis for selecting a timingreference.

In some priority rules (e.g., R1-2 a, R1-2 b in the table 700),different types of synchronization sources are organized into differentpriority groups (e.g., PG1, PG2, and the like). For each such priorityrule, the priority groups are listed from high priority to low priority(e.g., from PG1 to PG6 in R1-2 a).

In addition, in an example, within a same priority group, when multiplesynchronization sources are available, the one with a higher signalquality (e.g., measured by sidelink RSRP (S-RSRP)) can be selected.

Different priority rules can be configured to UEs having differentcapabilities. For example, the rules in the tables 600-700 can be usedby a NR only and NR V2X only UE. The rules in the tables 800-900 can beused by a NR/LTE (NR and LTE capable) and NR V2X only UE. The rules inthe tables 1000-1100 can be used by a NR only and NR V2X/LTE V2X UE. Therules in the tables 1200-1300 can be used by a NR/LTE and NR V2X/LTE V2XUE. The invention is not limited by these.

For UEs in an in-coverage status (e.g., within a coverage of a gNB, oreNB), or an our-of-coverage status (e.g., not within a coverage of a gNBor eNB), different priority rules can be applied. Accordingly, thepriority rules corresponding to the in-coverage and our-of-coveragestatus are separate into different tables in the examples of the tables600-1300.

The tables 600 and 700 include priority rules for NR only and NR V2Xonly UEs in in-coverage status and out-of-coverage status, respectively.The table 600 includes two rules, denoted by R1-1 a and R1-1 b, that aregNB-based and GNSS-based, respectively. The table 700 includes tworules, R1-2 a and R1-2 b, that are gNB-based and GNSS-based,respectively.

In the rule R1-1 a, when a UE is within a coverage (InC) of a cell thatoperates with a frequency the same as that of a sidelink of the UE, aPCell or SCell of the UE can be used as the synchronization source whenthe UE is in RRC connected mode. Or, a serving cell that the UE camps oncan be used as the synchronization source when the UE is in RRC idlemode. Or, SSs from a downlink frequency paired with the respectivesidelink frequency of the UE can be used as the timing reference. Whenthe UE is out of coverage (OoC) of the cell that operates with afrequency the same as that of a sidelink of the UE, a PCell the UE isconnected with or a serving cell the UE camps on can be used as asynchronization source. This PCell or serving cell can operate with afrequency different from that of the sidelink of the UE.

In the rule R1-1 b, one of a GNSS, a NR UE directly synchronized to theGNSS, and a gNB can be used as a synchronization source. The GNSS has ahighest priority while the gNB has a lowest priority.

In addition, NR_UE_(GNSS) denotes a NR UE synchronized to a GNSS.NR_UE_(gNB) denotes a NR UE synchronized to a gNB. NR_UE_(NR_UE-gNB)denotes a NR UE synchronized to a NR_UE_(gNB). NR_UE_(NR_UE-GNSS)denotes a NE UE synchronized to a NR_UE_(GNSS). Meanings of othernotations in the tables 600-1300 can be interpreted similarly.

The tables 800 and 900 include priority rules for NR/LTE and NR V2X onlyUEs in in-coverage status and out-of-coverage status, respectively.Compared with the tables 600 and 700, LTE related synchronizationsources (e.g., PCell/Serving cell of LTE, NR-UE_(NR_UE-eNB),NR_UE_(LTE_UE-GNSS)) are added to the rules.

The tables 1000 and 1100 include priority rules for NR only and NRV2X/LTE V2X UEs in in-overage status and out-of-coverage status,respectively. Compared with the tables 600 and 700, LTE UE relatedsynchronization sources (e.g., LTE_UE_(GNSS), LTE-UE_(eNB),LTE_UE_(other)) are added to the rules.

The tables 1200 and 1300 include priority rules for NR/LTE and NRV2X/LTE V2X UEs in in-overage status and out-of-coverage status,respectively. Compared with the tables 600 and 700, LTE and LTE UErelated synchronization sources are added to the rules.

As shown in the tables 700/900/1100/1300, the gNB or eNB based rules forout-of-coverage usage, R1-2 a (in the table 700), R2-2 a and R2-2 b (inthe table 900), R3-2 a and R3-2 b (in the table 1100), and R4-2 a andR4-2 b (in the table 1300), each include 6 priority groups from PG1 toPG6. Similar to the priority types from P1′ to P6′ listed in thesynchronization source priority rule 400 in FIG. 4, the priority groupPG1 includes UEs that are directly synchronized to a gNB or an eNB. Thepriority group PG2 includes UEs that are indirectly synchronized to agNB or an eNB. The priority group PG3 includes one or more GNSSs. Thepriority group PG4 includes UEs that are directly synchronized to aGNSS. The priority group PG5 includes UEs that are indirectlysynchronized to a GNSS. The priority group PG6 includes other UEs.Please note that in the tables 700/900/1100/1300, priority group PG0 areomitted for brevity. Similar to the priority type P0′ listed in thesynchronization source priority rule 400 in FIG. 4, the priority groupPG0 includes one or more eNB/gNB (to be synchronized by the UE).

As shown in the tables 700/900/1100/1300, the GNSS based rules forout-of-coverage usage, R1-2 b, R2-2 c, R3-2 c, and R4-2 c, each include4 priority groups from PG1 to PG4. Similar to the priority types from P0to P3 listed in the synchronization source priority rule 500 in FIG. 5,the priority group PG1 includes one or more GNSSs. The priority groupPG2 includes UEs directly synchronized to a GNSS. The priority group PG3includes UEs indirectly synchronized to a GNSS.

In some embodiments, beamforming sweeping is employed in sidelinktransmissions. Accordingly, SLSSs in the form of sidelinksynchronization signal blocks (S-SSBs) can be transmitted over beamstowards different directions during a beam sweeping to cover a cell. TheS-SSBs in the beam sweeping can be organized into an S-SSB burst. EachS-SSB can include an S-PSS, an S-SSS, a PSBCH, and a demodulationreference signal multiplexed with the PSBCH (PSBCH DMRS). The S-SSBburst can be transmitted periodically, for example, for every 5 ms, 10ms, 20 ms, and the like.

In addition, different numerologies may be employed for frequencies usedin sidelink transmissions. In an example, a default numerology or a setof numerologies for sidelink transmissions can be preconfigured to a UE(e.g., storage in a SIM module). Accordingly, the UE can search for aSLSS with the default numerology or one of the set of configurednumerologies. In an example, one or more numerologies can be signaledfrom a BS to a UE. Accordingly, the UE can search for a SLSS with thesignaled numerologies.

For different numerologies, different subcarrier spacings (e.g., 15 kHz,30 kHz, 60 kHz, and the like) can be used, and accordingly, differentnumber of slots can be included in a subframe of 1 ms. For example,corresponding to the subcarrier spacings 15 kHz, 30 kHz, 60 kHz, and 120kHz, each subframe can include 1, 2, 4, and 8 slots, respectively. Undersuch a configuration, for different numerologies, positions of S-SSBs ofan S-SSB burst over a subframe may be arranged differently for differentnumerologies.

In an embodiment, slot index information of an S-SSB corresponding to acertain numerology is signaled to a UE from a BS, or is carried in theS-SSB, such that the UE can determine which slot (e.g., indicated by aslot index) the respective S-SSB is positioned in. The S-SSB can alsocarry information of a system frame number (SFN) and subframe indexassociated with the S-SSB. Based on the information of the slot,subframe, and the SFN, the UE can determine timings of the respectiveS-SSB.

In an embodiment, number of bits used for indicating the slot index ofthe S-SSB depends on the numerology of the S-SSB. For example, for thenumerology of 120 kHz, there can be 8 slots in a subframe, while for thenumerology of 30 kHz, there can be 2 slots in a subframe. Accordingly, amaximum allowed positions for S-SSBs for the numerology of 120 kHz canbe more than that for the numerology of 30 kHz. Thus, more bits arepotentially needed for representing the slot indices for the numerologyof 120 kHz that for the numerology of 30 kHz. In an example, 1 bit, 2bits, and 4 bits are used for indicating slot indices for thenumerologies of 15 kHz, 30 kHz, and 60 kHz, respectively.

In an embodiment, the slot index information and the subframeinformation of an S-SSB is combined into one field and indicated by asame set of bits carried in the respective S-SSB.

In an embodiment, the slot index information is carried in an S-SSBusing sequences of SSS or PSBCH DMRS. For example, different sequencescan be selected or generated (e.g., initialized with bits of a slotindex) to indicate the slot index information.

In an embodiment, uplink frequencies of paired frequencies or frequencydivision duplex (FDD) frequencies are used for V2X communication. Tofacilitate a UE to perform sidelink synchronization, downlink frequencyinformation (e.g., an absolute radio frequency channel number (ARFCN))and/or band information is signaled to a UE or preconfigured to a UE(e.g., stored in a SIM). Based on those information, the UE can searchuplink frequencies corresponding to the known downlink frequencies toperform a synchronization process while in coverage or out of coverageof a BS.

In various embodiments, sidelink synchronization source priorityinformation for indicating a priority of a SLSS can be carried in theSLSS (e.g., S-SSB) in various ways. The sidelink synchronization sourcepriority information can include which type of top-priority source(e.g., GNSS, gNB, or eNB) is used, coverage status of the respective UE(e.g., in-coverage or out-of-coverage), whether no top-priority sourceis present (e.g., a cluster of UEs are synchronized to a timingreference determined autonomously be a UE), numbers of hops with respectto a top-priority source, and the like.

In an embodiment, part of the synchronization source priorityinformation (e.g. in-coverage or out-of-coverage) is included in a PSBCHof a SLSS. Accordingly, the PBSCH has to be decoded before a priority ofthe SLSS can be determined. In an embodiment, information bits of thesynchronization source priority information can be arranged in specialinput positions to a polar encoder when polar code is used for channelcoding a master information block (MIB) in a PSBCH. In this way,transmission reliability of the respective information bits can beimproved, and decoding of those information bits can be acceleratedwithout fully decoding the MIB in the PSBCH.

In an embodiment, instead of a PSBCH, the synchronization sourcepriority information is carried in an S-PSS, an S-SSS, or a PSBCH DMRSof an S-SSB, or a combination of two or three of an S-PSS, an S-SSS, ora PSBCH DMRS of an S-SSB. In this way, the synchronization sourcepriority information can be determined earlier without decoding thePSBCH.

In an embodiment, a sidelink ID for identifying sidelink unicast,groupcast, or broadcast is determined based on different in-coverage orout-of-coverage scenarios. For example, the sidelink ID can beconfigured to a Tx UE by RRC signaling (e.g., similar to assigning RNTI)or by storage in the Tx UE. The sidelink ID can be used to identify asidelink of unicast, groupcast, or broadcast when the Tx UE performssidelink transmissions. Or, in other words, the sidelink ID can be usedto identify communications between the UE and an individual UE, or agroup of UEs. During a sidelink transmission from the Tx UE, such asidelink ID can be explicitly carried in a control channel or implicitlycarried via CRC scrambling (e.g., a CRC scrambled by such an ID). A RxUE can know if the transmission is intended for the Tx UE or a groupincluding the Tx UE by detecting or decoding the ID. In addition, theIDs can also be used for other functions including scrambling control ordata information over a sidelink, or scrambling control or data DMRSover a sidelink.

The sidelink ID can be a L1-ID (physical layer ID) that is derived froma L2-ID or a higher layer ID, or configured by a higher layer.

In an example, when the Tx UE is in an out-of-coverage status without aserving cell (without presence of a BS) (e.g., the FIG. 3 example), thesidelink ID is used by itself. In contrast, when the Tx UE is anin-coverage status or an our-of-coverage status with a serving cell orBS present (e.g., the FIG. 2 example), the sidelink ID can be scrambledby a cell ID of the serving cell and the resulting ID can be used inplace of the sidelink ID.

FIG. 14 shows a sidelink data transmission process 1400 according to anembodiment of the disclosure. The BS 101 and the UEs 102-103 in FIG. 1Aand FIG. 1B are used as examples for explanation of the process 1400.The process 1400 can be performed by the BS 101 to dynamically selectthe BS 101 or the Tx UE 102 to perform a retransmission of sidelink datatransmitted from the Tx UE 102 to the Rx UE 103. The selection can bebased on channel conditions of the sidelink 120 between the Tx UE 102and the Rx UE 103, and the Uu link 112 between the BS101 and the Rx UE103. The process can start from S1401, and proceed to S1410.

At S1410, the sidelink data can be received at the BS 101. The sidelinkdata can be transmitted from the Tx UE 102 to the Rx UE 103 over thesidelink 120. As the sidelink 120 operates over the same frequency asthe uplink between the Tx UE 102 and the BS 101, the BS 101 can detectand decode a signal of the sidelink data when transmission power of thesidelink data is suitable, and a channel condition over the uplink isabove a threshold.

At the S1420, a NACK can be received from the Rx UE 103 at the BS 101.For example, when reception of the sidelink data over the sidelink 120is failed, the Rx UE 103 can transmit the NACK over the sidelink 120.Because the uplink between the Rx UE 103 and the BS 101 shares a samefrequency as the sidelink 120, the BS 101 can receive the NACK whentransmission power of the NACK is suitable and a channel condition overthe uplink between the Rx UE 103 and the BS 101 is above a threshold.

At S1430, the BS 101 can make a decision to select one or both of the BS101 and the Tx UE 102 to perform a retransmission of the sidelink datain response to receiving the NACK. The selection can be based on channelconditions of the sidelink 120 and the Uu link 112, for example, asmeasured by the Rx UE 103. In other examples, other factors may beconsidered for the selection.

At 1440, when the BS is selected, the BS 101 can retransmit the sidelinkdata received at S1410 to the Rx UE 103 over the Uu link 112.

When the Tx UE is selected, the BS 101 can transmit a sidelink grant tothe Tx UE 102 to allocate radio resources of the sidelink 120 for aretransmission to be performed by the Tx UE 102. Subsequently, the Tx UE102 can retransmit the sidelink data to the Rx UE 103. The process 1400can proceed to S1499 and terminates at S1499.

FIG. 15 shows a sidelink data transmission process 1500 according to anembodiment of the disclosure. The BS 101 and the UEs 102-103 in FIG. 1Aand FIG. 1B are used as examples for explanation of the process 1500.The process 1500 can be performed by the Rx UE 103 for reception ofsidelink data without receiving resource allocation information (e.g.,carried in a sidelink DCI in a PSCCH from the Tx UE 102).

The process 1500 can start from S1501, and proceeds to S1510.

At S1510, a sidelink grant can be received from the BS 101 at the Rx UE103 over the Uu link 112. The sidelink grant can indicate radioresources for transmitting the sidelink data over the sidelink 120 fromthe Tx UE 102 to the Rx UE 103. For example, the sidelink grant can becarried in a DCI having a CRC scrambled with a common ID known to boththe Tx UE 102 and the Rx UE 103. Accordingly, the Tx UE 102 and the RxUE 103 can detect and decode a PDCCH carrying the DCI, and obtain thesidelink grant.

At S1520, the sidelink data can be detected over the sidelink 120 at theRx UE 103 according to the sidelink grant. For example, based on thesidelink grant, the Rx UE 103 can know time-frequency domain locationsof the radio resources used for transmitting the sidelink data, and arespective MCS. Under such a configuration, transmission of a PSCCH forscheduling resources for transmitting the sidelink data can be avoided.The process 1500 can proceed to S1599 and terminates at S1599.

FIG. 16 shows a sidelink synchronization process 1600 according to anembodiment of the disclosure. The process 1600 can be performed by a UEto select a reference timing and subsequently synchronize to theselected reference timing. The process 1600 can start from S1601, andproceed to S1610.

At S1610, a sidelink synchronization source priority rule can bereceived at the UE. The received rule can be a gNB/eNB-based rule (e.g.,the FIG. 4 example), or a GNSS-based rule (e.g., the FIG. 5 example),depending a deployment preference of a network operator. The rule can bereceived from a BS, for example, by RRC signaling or system informationbroadcasting. Alternatively, the rule can be received from a SIM or UICCmodule, or a memory at the UE. In addition, the rule can be representedby a sidelink synchronization source priority configuration (or rule)derived from or compliant with the rule, similar to the examples show inFIG. 6-FIG. 13, considering capabilities of the UE.

At S1620, a timing reference with a highest priority among availablesidelink synchronization sources is selected according to the rulereceived at the S1610. For example, based on a configuration receivedfrom a BS or preconfigured to the UE (e.g., stored in a SIM), the UE canknow one or more frequencies over which SSs (e.g., an SS from a BS, orSLSS) are transmitted. Accordingly, the UE can tune to those frequenciesto detect SSs which can be used as timing references. Those timingreferences (the SSs) can each carry sidelink synchronization priorityinformation.

According to priorities of different types of synchronization sourcesspecified in the rule, the UE may investigate available SSs near the UEone by one (or more than one at a time) according to an order, andsearch for an SS with the highest priority. Take the rule of FIG. 4 asan example, the UE can first search for an SS from a gNB or eNB of thepriority P0. If no gNB or eNB is found (e.g., no SS from a gNB or eNBexists, or a signal quality of an SS received from a BS is below athreshold), the UE can select a SLSS of a UE directly synchronized to agNB or an eNB among available SLSSs when the SLSS is available and has aquality above a threshold. This process can continue until an availableSLSS with a highest priority is found.

At S1630, a transmission timing can be determined according to theselected timing reference.

For example, the UE can adjust a local timing (e.g., a clock) to alignwith the selected timing reference, and transmit a SLSS according to thelocal timing. The process 1600 can proceed to S1699 and terminate atS1699.

FIG. 17 shows an exemplary apparatus 1700 according to embodiments ofthe disclosure. The apparatus 1700 can be configured to perform variousfunctions in accordance with one or more embodiments or examplesdescribed herein. Thus, the apparatus 1700 can provide means forimplementation of mechanisms, techniques, processes, functions,components, systems described herein. For example, the apparatus 1700can be used to implement functions of the UEs102-103 or the BS 101 invarious embodiments and examples described herein. The apparatus 1700can include a general purpose processor or specially designed circuitsto implement various functions, components, or processes describedherein in various embodiments. The apparatus 1700 can include processingcircuitry 1710, a memory 1720, and a radio frequency (RF) module 1730.

In various examples, the processing circuitry 1710 can include circuitryconfigured to perform the functions and processes described herein incombination with software or without software. In various examples, theprocessing circuitry 1710 can be a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), programmable logicdevices (PLDs), field programmable gate arrays (FPGAs), digitallyenhanced circuits, or comparable device or a combination thereof.

In some other examples, the processing circuitry 1710 can be a centralprocessing unit (CPU) configured to execute program instructions toperform various functions and processes described herein. Accordingly,the memory 1720 can be configured to store program instructions. Theprocessing circuitry 1710, when executing the program instructions, canperform the functions and processes. The memory 1720 can further storeother programs or data, such as operating systems, application programs,and the like. The memory 1720 can include non-transitory storage media,such as a read only memory (ROM), a random access memory (RAM), a flashmemory, a solid state memory, a hard disk drive, an optical disk drive,and the like.

In an embodiment, the RF module 1730 receives a processed data signalfrom the processing circuitry 1710 and converts the data signal tobeamforming wireless signals that are then transmitted via antennaarrays 1740, or vice versa. The RF module 1730 can include a digital toanalog convertor (DAC), an analog to digital converter (ADC), afrequency up convertor, a frequency down converter, filters andamplifiers for reception and transmission operations. The RF module 1730can include multi-antenna circuitry for beamforming operations. Forexample, the multi-antenna circuitry can include an uplink spatialfilter circuit, and a downlink spatial filter circuit for shiftinganalog signal phases or scaling analog signal amplitudes. The antennaarrays 1740 can include one or more antenna arrays.

The apparatus 1700 can optionally include other components, such asinput and output devices, additional or signal processing circuitry, andthe like. Accordingly, the apparatus 1700 may be capable of performingother additional functions, such as executing application programs, andprocessing alternative communication protocols.

The processes and functions described herein can be implemented as acomputer program which, when executed by one or more processors, cancause the one or more processors to perform the respective processes andfunctions. The computer program may be stored or distributed on asuitable medium, such as an optical storage medium or a solid-statemedium supplied together with, or as part of, other hardware. Thecomputer program may also be distributed in other forms, such as via theInternet or other wired or wireless telecommunication systems. Forexample, the computer program can be obtained and loaded into anapparatus, including obtaining the computer program through physicalmedium or distributed system, including, for example, from a serverconnected to the Internet.

The computer program may be accessible from a computer-readable mediumproviding program instructions for use by or in connection with acomputer or any instruction execution system. The computer readablemedium may include any apparatus that stores, communicates, propagates,or transports the computer program for use by or in connection with aninstruction execution system, apparatus, or device. Thecomputer-readable medium can be magnetic, optical, electronic,electromagnetic, infrared, or semiconductor system (or apparatus ordevice) or a propagation medium. The computer-readable medium mayinclude a computer-readable non-transitory storage medium such as asemiconductor or solid state memory, magnetic tape, a removable computerdiskette, a random access memory (RAM), a read-only memory (ROM), amagnetic disk and an optical disk, and the like. The computer-readablenon-transitory storage medium can include all types of computer readablemedium, including magnetic storage medium, optical storage medium, flashmedium, and solid state storage medium.

While aspects of the present disclosure have been described inconjunction with the specific embodiments thereof that are proposed asexamples, alternatives, modifications, and variations to the examplesmay be made. Accordingly, embodiments as set forth herein are intendedto be illustrative and not limiting. There are changes that may be madewithout departing from the scope of the claims set forth below.

1. A method, comprising: selecting at a user equipment (UE) a timingreference with a highest priority among available sidelinksynchronization timing references according to a sidelinksynchronization source priority rule that includes different types ofsidelink synchronization timing references each having a priority; anddetermining at the UE a transmission timing according to the selectedtiming reference, wherein when the different types of sidelinksynchronization timing references are gNB or eNB based timingreferences, the different types of sidelink synchronization timingreferences include the following types of sidelink synchronizationtiming references listed from high priority to low priority: P0′: gNB oreNB, P1′: UE directly synchronized to gNB or eNB, P2′: UE in directlysynchronized to gNB or eNB, P3′: global navigation satellite system(GNSS), P4′: UE directly synchronized to GNSS, P5′: UE indirectlysynchronized to GNSS, P6′: remaining UEs.
 2. The method of claim 1,wherein when the different types of sidelink synchronization timingreferences are GNSS based references, the different types of sidelinksynchronization timing references include the following types ofsidelink synchronization timing references listed from high priority tolow priority: P0: GNSS, P1: UE directly synchronized to GNSS, P2: UEindirectly synchronized to GNSS, P3: remaining UEs.
 3. The method ofclaim 1, wherein the selecting includes: receiving a sidelinksynchronization signal (SLSS) carrying information of a slot index. 4.The method of claim 3, wherein a number of bits representing the slotindex depends on a numerology of the SLSS.
 5. The method of claim 3,where the SLSS includes a sidelink secondary synchronization signal(S-SSS) and a demodulation reference signal (DMRS), and a sequence ofthe S-SSS, a sequence of the DMRS, or the sequences of the S-SSS and theDMRS in combination, represent the slot index.
 6. The method of claim 3,wherein information of the slot index and a subframe index is combinedas one field and carried in the SLSS.
 7. The method of claim 1, whereinthe selecting includes: receiving a SLSS carrying sidelinksynchronization source priority information, wherein the sidelinksynchronization source priority information is carried in signalsincluded in the SLSS other than a sidelink physical broadcast channel(PBCH).
 8. The method of claim 1, wherein the selecting includes:receiving a SLSS carrying sidelink synchronization source priorityinformation in a sidelink PBCH, wherein bits of the sidelinksynchronization source priority information are arranged at input bitpositions of a polar encoder when information bits of the sidelink PBCHare encoded with the polar encoder, such that the bits of the sidelinksynchronization source priority information can be obtained withoutfully decoding the sidelink PBCH.
 9. The method of claim 1, furthercomprising: receiving a sidelink identifier (ID) for identifyingsidelink unicast, groupcast, or broadcast, and performing a sidelinktransmission using the sidelink ID scrambled with a cell ID of a servingcell when the UE is within or out of coverage of the serving cell.
 10. Auser equipment (UE), comprising circuitry configured to: select a timingreference with a highest priority among available sidelinksynchronization timing references according to a sidelinksynchronization source priority rule that includes different types ofsidelink synchronization timing references each having a priority; anddetermine a transmission timing according to the selected timingreference, wherein when the different types of sidelink synchronizationtiming references are gNB or eNB based timing references, the differenttypes of sidelink synchronization timing references include thefollowing types of sidelink synchronization timing references listedfrom high priority to low priority: P0′: gNB or eNB, P1′: UE directlysynchronized to gNB or eNB, P2′: UE in directly synchronized to gNB oreNB, P3′: global navigation satellite system (GNSS), P4′: UE directlysynchronized to GNSS, P5′: UE indirectly synchronized to GNSS, P6′:remaining UEs.
 11. The UE of claim 10, wherein when the different typesof sidelink synchronization timing references are GNSS based references,the different types of sidelink synchronization timing referencesinclude the following types of sidelink synchronization timingreferences listed from high priority to low priority: P0: GNSS, P1: UEdirectly synchronized to GNSS, P2: UE indirectly synchronized to GNSS,P3: remaining UEs.
 12. The UE of claim 10, wherein the circuitry isconfigured to: receive a sidelink synchronization signal (SLSS) carryinginformation of a slot index.
 13. The UE of claim 12, wherein a number ofbits representing the slot index depends on a numerology of the SLSS.14. The UE of claim 12, where the SLSS includes a sidelink secondarysynchronization signal (S-SSS) and a demodulation reference signal(DMRS), and a sequence of the S-SSS, a sequence of the DMRS, or thesequences of the S-SSS and the DMRS in combination, represent the slotindex.
 15. The UE of claim 12, wherein information of the slot index anda subframe index is combined as one field and carried in the SLSS. 16.The UE of claim 10, wherein the circuitry is configured to: receive aSLSS carrying sidelink synchronization source priority information,wherein the sidelink synchronization source priority information iscarried in signals included in the SLSS other than a sidelink physicalbroadcast channel (PBCH).
 17. The UE of claim 10, wherein the circuitryis configured to: receive a SLSS carrying sidelink synchronizationsource priority information in a sidelink PBCH, wherein bits of thesidelink synchronization source priority information are arranged atinput bit positions of a polar encoder when information bits of thesidelink PBCH are encoded with the polar encoder, such that the bits ofthe sidelink synchronization source priority information can be obtainedwithout fully decoding the sidelink PBCH.
 18. The UE of claim 10,wherein the circuitry is configured to: receive a sidelink identifier(ID) for identifying sidelink unicast, groupcast, or broadcast, andperform a sidelink transmission using the sidelink ID scrambled with acell ID of a serving cell when the UE is within or out of coverage ofthe serving cell.
 19. A non-transitory computer-readable medium storinga program that is executable by a processor to perform a method, themethod comprising: selecting at a user equipment (UE) a timing referencewith a highest priority among available sidelink synchronization timingreferences according to a sidelink synchronization source priority rulethat includes different types of sidelink synchronization timingreferences each having a priority; and determining at the UE atransmission timing according to the selected timing reference, whereinwhen the different types of sidelink synchronization timing referencesare gNB or eNB based timing references, the different types of sidelinksynchronization timing references include the following types ofsidelink synchronization timing references listed from high priority tolow priority: P0′: gNB or eNB, P1′: UE directly synchronized to gNB oreNB, P2′: UE in directly synchronized to gNB or eNB, P3′: GNSS, P4′: UEdirectly synchronized to GNSS, P5′: UE indirectly synchronized to GNSS,P6′: remaining UEs.
 20. The non-transitory computer-readable medium ofclaim 19, wherein when the different types of sidelink synchronizationtiming references are global navigation satellite system (GNSS) basedreferences, the different types of sidelink synchronization timingreferences include the following types of sidelink synchronizationtiming references listed from high priority to low priority: P0: GNSS,P1: UE directly synchronized to GNSS, P2: UE indirectly synchronized toGNSS, P3: remaining UEs.